1. Field of the Invention
This invention relates to a solid laser. This invention particularly relates to a solid laser, wherein an anisotropic laser crystal is employed as a laser medium, and either one of two laser beams having different directions of polarization and different wavelengths is selectively radiated out of the solid laser.
2. Description of the Prior Art
Solid lasers have been proposed in, for example, Japanese Unexamined Patent Publication No. 62(1987)-189783. The proposed solid lasers comprise a laser crystal, to which a rare earth element, such as neodymium (Nd), has been added. The laser crystal is pumped by a semiconductor laser, or the like. Basically, the solid laser comprises the laser crystal, a pumping means, which causes a pumping beam to impinge upon the laser crystal, and a resonator mirror, which oscillates a solid laser beam having been produced by the laser crystal.
In solid lasers of this type, anisotropic crystals, such as LiYF.sub.4 (hereinafter referred to as YLF), are often employed as laser crystals. In such cases, in general, two beam radiation lines occur in a single oscillation wavelength band. For example, in cases where Nd-doped YLF (hereinafter referred to as Nd:YLF) is employed as the laser crystal, a laser beam having a wavelength of 1,053 nm, which is the so-called .sigma.-polarized light, and a laser beam having a wavelength of 1,047 nm, which is the so-called .pi.-polarized light, can be radiated out of the solid laser in an oscillation wavelength band of 1.0 .mu.m. Also, in an oscillation wavelength band of 1.3 .mu.m, a beam radiation line, in which a laser beam having a wavelength of 1,313 nm (.sigma.-polarized light) is radiated out of the solid laser, and a beam radiation line, in which a laser beam having a wavelength of 1,321 nm (.pi.-polarized light) is radiated out of the solid laser, can occur. This is because ordinary rays oscillate in one of the two beam radiation lines, and extraordinary rays oscillate in the other beam radiation line.
Therefore, in cases where an anisotropic laser crystal is employed as the laser crystal, in general, it is necessary for a countermeasure to be devised such that a single desired beam radiation line may be obtained. For example, such a technique is described in Optics Letters, Vol. 11, No. 4, April 1986. With the described technique, a Brewster plate is inserted into a resonator for a solid laser, in which an anisotropic laser crystal is employed. The Brewster plate is rotated 90.degree., and one of two beam radiation lines is thereby selected.
However, the conventional solid laser utilizing the Brewster plate has the problems in that a laser beam cannot be radiated with a high efficiency out of the solid laser due to large loss occurring from the insertion of the Brewster plate into the resonator.
Also, with the conventional solid laser utilizing the Brewster plate, a complicated mechanism is required to rotate the Brewster plate, and re-adjustments for the resonator mirror must be carried out as the Brewster plate is rotated. Therefore, the conventional solid laser utilizing the Brewster plate has the drawbacks in that the structure cannot be kept simple, and adjustments cannot be carried out easily.